Former champion Brooke Henderson edged into a share of the Women's PGA Championship lead as Lydia Ko surged into contention Friday.
Henderson – the 2016 winner – carded a one-under 71 in the second round at Kemper Lakes Golf Club to move into six under and a three-way tie for a one-stroke advantage.
The Canadian is joined atop the leaderboard by Ryu So-yeon (69) and first-round leader Park Sung-hyun (72).
But it was Ko – the 21-year-old New Zealander – who starred, firing a six-under 66 in Illinois.
Ko, a two-time major champion, mixed eight birdies with two bogeys to move into four under, sitting in a tie for fifth.
Since going on a run of five straight top-three finishes – including her two wins – at majors in 2015 and 2016, Ko has managed to do that just once more in her past nine.
Carlota Ciganda (69) is in outright fourth at five under, while Annie Park (69) and Moriya Jutanugarn (72) are alongside Ko in a tie for fifth.
Defending champion Danielle Kang bounced back with a three-under 69 to be at two under overall.
Jaye Marie Green made a triple bogey at the par-three 17th to be at three under, having appeared set to be in a tie for the lead.
World number one Inbee Park missed the cut, following her opening 73 with a four-over 76.